MicroRNA (miR) regulation and protein destruction are two common themes in spatiotemporal regulation of cellular networks. These post-transcriptional mechanisms fine tune protein action within cells and play important roles in diverse processes among eukaryotes including oncogenic signaling, cell cycle regulation, circadian rhythms, pathogen response and cell specification. Both the miR and ubiquitin- proteasome machineries are highly conserved between eukaryotes, and their study in plants has some marked advantages over other model systems. For instance, plant miRs target unique, easily identifiable mRNAs, minimizing the possibility of indirect effects. This is in contrast to metozoan systems where miR target prediction is still difficult. Plants also have a greatly expanded number of F-box proteins, molecules controlling a major route of protein degradation. This expansion may have contributed to increased cell type and process specialization for these proteins. As the specificity of F-box proteins is not well understood, studies in the model plant Arabidopsis provide a unique opportunity to examine how accurate substrate targeting occurs. This proposal is focused on the function and regulation of the Arabidopsis ULO (UNUSUAL LATERAL ORGANS) protein, a Kelch domain containing F-box protein that is post-transcriptionally regulated by two miRs (miR394a/b). Specifically, this project aims to (1) determine the composition of ULO-specific protein degradation complex(es), (2) examine the transcriptional and post-transcriptional control of ULO, and (3) determine the contribution of ULO to organogenesis. These goals will be accomplished by employing biochemistry, proteomics, genetics, and microscropy. ULO provides a unique opportunity to study the interplay between the deeply conserved miR- and proteasome-mediated regulatory mechanisms in a genetically tractable, multicellular organism. PUBLIC HEALTH RELEVANCE: This research is focused on plant development and utilizes Arabidopsis thaliana (mouse-ear cress), which is a key model organism and a close relative of numerous crop species such as canola, broccoli and papaya. The goal is to determine the role of a protein, UNUSUAL LATERAL ORGANS (ULO), in organ patterning during embryo, leaf and flower formation. ULO-related proteins are found in many organisms, including humans, and this large family of proteins can play important roles in many aspect of human disease, including cancer and diabetes.